有没有办法加快这个查询？

Question

我怀疑有一种方法可以使其更快，但它超出了我的MySQL限制。

我有一张表，它由从某些传感器收集的数据组成，按活动为基础以1Hz的比率进行活动。表列是activityId，transducerId（数据来自哪个传感器），传感器正在报告的值以及时间戳。一个给定的活动可以有0 - 24个传感器。

我需要一个新的表命名为含该传感器的数据的每个传感器的列，和：等（取决于传感器的数量给予或采取行）该数据的

一个第二看起来日期时间列。例如：

目前我得到这个表有一个很长的一系列查询和连接。下面是我使用的查询：

SELECT cd.calculatedValue AS `301`, q1.`302` , q2.`303` , q3.`304` , q4.`305` , q5.`306` , q6.`307` , q7.`308` , q8.`309` , q9.`310` , q10.`311` , q11.`312` , q12.`313` , q13.`314` , cd.`datetime` 
FROM 
data cd 
JOIN 
(SELECT `calculatedValue` AS `302`, `datetime` FROM `data` WHERE `activityId` = 74 AND `transducerId` = 302) AS q1 
ON cd.`datetime` = q1.`datetime` 
JOIN 
(SELECT `calculatedValue` AS `303`, `datetime` FROM `data` WHERE `activityId` = 74 AND `transducerId` = 303) AS q2 
ON cd.`datetime` = q2.`datetime` 
JOIN 
(SELECT `calculatedValue` AS `304`, `datetime` FROM `data` WHERE `activityId` = 74 AND `transducerId` = 304) AS q3 
ON cd.`datetime` = q3.`datetime` 
JOIN 
(SELECT `calculatedValue` AS `305`, `datetime` FROM `data` WHERE `activityId` = 74 AND `transducerId` = 305) AS q4 
ON cd.`datetime` = q4.`datetime` 
JOIN 
(SELECT `calculatedValue` AS `306`, `datetime` FROM `data` WHERE `activityId` = 74 AND `transducerId` = 306) AS q5 
ON cd.`datetime` = q5.`datetime` 
JOIN 
(SELECT `calculatedValue` AS `307`, `datetime` FROM `data` WHERE `activityId` = 74 AND `transducerId` = 307) AS q6 
ON cd.`datetime` = q6.`datetime` 
JOIN 
(SELECT `calculatedValue` AS `308`, `datetime` FROM `data` WHERE `activityId` = 74 AND `transducerId` = 308) AS q7 
ON cd.`datetime` = q7.`datetime` 
JOIN 
(SELECT `calculatedValue` AS `309`, `datetime` FROM `data` WHERE `activityId` = 74 AND `transducerId` = 309) AS q8 
ON cd.`datetime` = q8.`datetime` 
JOIN 
(SELECT `calculatedValue` AS `310`, `datetime` FROM `data` WHERE `activityId` = 74 AND `transducerId` = 310) AS q9 
ON cd.`datetime` = q9.`datetime` 
JOIN 
(SELECT `calculatedValue` AS `311`, `datetime` FROM `data` WHERE `activityId` = 74 AND `transducerId` = 311) AS q10 
ON cd.`datetime` = q10.`datetime` 
JOIN 
(SELECT `calculatedValue` AS `312`, `datetime` FROM `data` WHERE `activityId` = 74 AND `transducerId` = 312) AS q11 
ON cd.`datetime` = q11.`datetime` 
JOIN 
(SELECT `calculatedValue` AS `313`, `datetime` FROM `data` WHERE `activityId` = 74 AND `transducerId` = 313) AS q12 
ON cd.`datetime` = q12.`datetime` 
JOIN 
(SELECT `calculatedValue` AS `314`, `datetime` FROM `data` WHERE `activityId` = 74 AND `transducerId` = 314) AS q13 
ON cd.`datetime` = q13.`datetime` 
WHERE cd.`activityId` = 74 AND cd.`transducerId` = 301 

这发生在短短几分钟内数据的很长一段时间，切实会有数据的时间在表中，还有多达10多个传感器。

有没有更好的方法来做这个查询？

非常感谢。

Answer 1

那些派生表将会在性能方面与你的午餐盒一起吃午餐。这些内联视图查询会运行并物化为临时MyISAM表，然后外部查询引用临时MyISAM表（这些表未编制索引）来执行所有联接操作。

作为替代方案，考虑在表中使用一个镜头，获得几乎相同的结果。 （在你的查询，如果日期时间行被用于任何换能器的“失踪”，不返回任何一行。

考虑使用GROUP BY操作，这MySQL可能能够使用合适的索引优化。

举个例子，像这样：

SELECT d.datetime 
    , MAX(IF(d.transducerId = 301,d.calculatedValue,NULL)) AS `301` 
    , MAX(IF(d.transducerId = 302,d.calculatedValue,NULL)) AS `302` 
    , MAX(IF(d.transducerId = 302,d.calculatedValue,NULL)) AS `302` 
    , MAX(IF(d.transducerId = 303,d.calculatedValue,NULL)) AS `303` 
    , MAX(IF(d.transducerId = 304,d.calculatedValue,NULL)) AS `304` 
    , MAX(IF(d.transducerId = 305,d.calculatedValue,NULL)) AS `305` 
    , MAX(IF(d.transducerId = 305,d.calculatedValue,NULL)) AS `306` 
    , MAX(IF(d.transducerId = 305,d.calculatedValue,NULL)) AS `307` 
    , MAX(IF(d.transducerId = 305,d.calculatedValue,NULL)) AS `308` 
    , MAX(IF(d.transducerId = 305,d.calculatedValue,NULL)) AS `309` 
    , MAX(IF(d.transducerId = 305,d.calculatedValue,NULL)) AS `310` 
    , MAX(IF(d.transducerId = 305,d.calculatedValue,NULL)) AS `311` 
    , MAX(IF(d.transducerId = 305,d.calculatedValue,NULL)) AS `312` 
    , MAX(IF(d.transducerId = 305,d.calculatedValue,NULL)) AS `313` 
    , MAX(IF(d.transducerId = 305,d.calculatedValue,NULL)) AS `314` 
    FROM `data` d 
WHERE d.activityId = 74 
GROUP BY d.datetime 

（您可以将d.datetime移动到SELECT列表的末尾，我通常有GROUP BY列第一。）

如果有并不是一个合适的指数，th查询是否会像一条沉重的货运列车一样汹涌澎湃地吹着烟雾，挣扎着陡峭的等级。

此查询的最合适的指数很可能将是

(activityID,datetime,transducerId,calculatedValue)

如果这是一个InnoDB表，并在集群重点龙头列(activityID,datetime)，这将是足够的。

理想情况下，此查询的EXPLAIN输出在Extra列中显示“使用where; using index”。我们绝对不想在EXPLAIN中看到的是“使用文件”操作，或任何派生表，我们可以帮助它。

---

该查询与原始数据略有不同，如果特定日期时间某个特定传感器的某一行“缺失”，则该查询将返回该日期时间的一行，但“缺失”传感器的空值将返回，原始查询将忽略整行。

---

如果你确实想一起去JOIN操作，那么相当于不利用内嵌的观点会比原来更高效，虽然可能效率不高的GROUP BY查询（在我的答案以上）。

SELECT cd301.datetime 
    , cd301.calculatedValue AS `301` 
    , cd302.calculatedValue AS `302` 
    , cd303.calculatedValue AS `303` 
    , cd304.calculatedValue AS `304` 
    , cd305.calculatedValue AS `305` 
    , cd306.calculatedValue AS `306` 
--  , cd307.calculatedValue AS `307` 
--  ... 
--  , cd314.calculatedValue AS `314` 
    FROM `data` cd301 
    JOIN `data` cd302 
    ON cd302.activityId = cd301.activityId 
    AND cd302.datetime  = cd301.datetime 
    AND cd302.transducerId = 302 
    JOIN `data` cd303 
    ON cd303.activityId = cd301.activityId 
    AND cd303.datetime  = cd301.datetime 
    AND cd303.transducerId = 303 
    JOIN `data` cd304 
    ON cd304.activityId = cd301.activityId 
    AND cd304.datetime  = cd301.datetime 
    AND cd304.transducerId = 304 
    JOIN `data` cd305 
    ON cd305.activityId = cd301.activityId 
    AND cd305.datetime  = cd301.datetime 
    AND cd305.transducerId = 305 
    JOIN `data` cd306 
    ON cd306.activityId = cd301.activityId 
    AND cd306.datetime  = cd301.datetime 
    AND cd306.transducerId = 306 
WHERE cd301.transducerId = 301 

很明显，这需要扩展到307,308，... 314遵循相同的模式。

此外，此JOIN方法可能与GROUP BY等效，甚至更快，但与单行GROUP BY计划相比，EXPLAIN将具有更多的行数。